An image input from an image input unit is divided into a ground portion and a document other than the ground portion in an image divider, and a latent image is extracted from a ground pattern included in the ground portion in a latent image extractor. Lightness of the latent image is corrected to be closer to lightness of the document in a lightness corrector, and a color of the latent image is converted into a gradation A in a latent image converter. A color of the document is converted in a document converter into a gradation B including the same plurality of colors as gradation A and having a different manner of gradation. The latent image and the document are integrated in an image integrator to generate an image including the latent image and the document having similar lightness with a difference in hues superposed on each other.